The present invention relates generally to a protection circuit for electrical apparatus, and more particularly, to an apparatus and system for detecting when silicon controlled rectifier (SCR) control elements in solid-state power supply equipment have failed.
SCRs are known in the power electronics industry for their use as high power and current switching devices. Such devices have found wide application in solid-state control systems where it is desired to control the voltage, current, and/or power supplied to a load. Generally, in single phase or poly-phase systems, one or more SCRs can be connected in series with a load to rectify a.c. to d.c. and control power to the d.c. load. SCRs can be further connected in a back to back relationship to an alternating current (AC) supply source, to supply a.c. current, voltage, or power to an a.c. load. Typically, SCRs are controlled to trigger into conduction at an appropriate time in the positive and negative half cycles of the AC supply. The time between the zero crossing of the AC supply waveform and the moment at which the SCRs are driven into conduction is commonly referred to as the firing angle of the SCR. Adjusting the firing angle provides a means for controlling the voltage, current, and/or power delivered to the load.
Prior art includes different methods of detecting operating conditions, including shorted SCRs, in a power supply source. As an example, one method detects a shorted SCR by using an amplitude detection circuit. Under such a short circuit condition, the faulted SCR causes the three-phase line or circuit to, in effect lose two phases and operate in a single-phase mode. Since the amplitude of the two phase voltages approach xc2xd amplitude, such faulted operation can be amplitude detected and the power supply disconnected from the three-power line.
A typical three-phase power supply includes a three-phase transformer having three primary and three secondary windings. The secondary windings provide three phase input voltages to a six SCR bridge. In the single-phase mode, two of the secondary voltages approach one-half of the amplitude of a third secondary voltage. The amplitude detection circuit is configured to detect the reduced amplitude of one or more of three secondary voltages of the three-phase transformer. The amplitude detection circuit is connected to the input of the SCR bridge and initiates a control signal that interrupts the input line power to the power source. The amplitude detection circuit monitors the three secondary voltages which are half-wave rectified, scaled by a voltage divider, filtered by a capacitor and compared to a fixed d.c. voltage by three voltage comparators. Outputs from the three voltage comparators are connected in an OR circuit such that, if the secondary inputs are less than some predetermined threshold, the output of the three input OR circuit is switched to a 0 Vdc output. The switching of the OR output to 0 Vd.c. causes the power source""s three-phase line contactor to open and the power source to be de-energized.
Problems exist with the current state of the art devices and methods used for amplitude detecting. Amplitude detection methods using OR circuit connections can fail to provide a proper signal indicating when a short circuit condition occurs. Variations in the secondary voltages due to voltage spikes in the primary windings or other fluctuations can cause false signals to the amplitude detection circuit resulting in unnecessary shut down of the power supply.
Under a shorted or faulted SCR condition, the current delivered to the transformer is uncontrolled and limited only by the voltage source and the impedance of the voltage source. The extremely high current delivered under such a condition, for multiple AC cycles, can cause overheating and result in damage to the power supply equipment.
There is a need for an apparatus and system capable of reliably sensing an SCR short circuit in solid-state power supply equipment. It would therefore be desirable to have an apparatus configured to detect a short circuit condition of an SCR and interrupt current flow to the solid-state power supply equipment in response thereto.
The present invention is directed to an apparatus and system to reliably sense an SCR short circuit in solid-state power supply equipment and output a signal or pulse capable of preventing extremely high short-current flow through the equipment to overcome the aforementioned concerns.
The present invention is directed to a protection circuit configured to prevent overheating of solid-state power supply equipment, such as a welder, during an occurrence of a short circuit condition in the control elements of the equipment. The circuit includes a shorted SCR detector circuit capable of detecting a short circuit condition of an SCR in the power supply equipment. The shorted SCR detector circuit monitors the transformer secondary voltages and generates an output signal or pulse, if a short circuit condition one one or more of the SCRs is detected. A control circuit receives the output signal or pulse from the shorted SCR detector circuit and generates an output capable of preventing current flow to the power supply equipment.
In accordance with one aspect of the present invention, an apparatus to sense an SCR short circuit failure includes a detector circuit configured to sense each voltage of a poly-phase input, such as a three-phase input, and generate a fault signal. The fault signal is generated upon substantially simultaneous detection of a zero-cross voltage condition for each phase of the poly-phase input. The invention also has a control circuit configured to receive the fault signal of the detection circuit and generate an output to interrupt current flow to the apparatus.
In accordance with another aspect of the present invention, a system to detect a short circuit of an SCR in a welder includes an electric current regulator for controlling electric current flow in the welder. The invention also includes a three-phase transformer connected to the electric current regulator. The electric current regulator is configured to receive a three-phase power supply input. The three phase transformer and the shorted SCR detecting means are capable of transforming the three-phase power supply voltages into three single phase detector inputs, wherein each of the single phase voltages lead or lag other phases under normal operating conditions. The system further includes a plurality of SCRs configured to receive the three single-phase voltages and generate a welder output. Additionally, the system has a detector circuit configured to receive and monitor the three single phase voltages and create a fault condition output signal capable of preventing electric current flow to the electric current regulator, if any one of the SCRs is shorted.
In accordance with the process of the present invention, a method to sense an SCR short includes the steps of sensing input voltage to a plurality of SCRs and comparing the input voltage to each SCR. If each input voltage is within a given range at a substantially similar time, then the method interrupts the line input voltages to the plurality of SCRs preventing short-circuit current flow through the SCRs.
In a further aspect of the present invention, an apparatus for detecting an SCR short circuit condition in a welder includes a means for controlling electric current flow through at least one silicon controlled rectifier and a means for sensing instantaneous voltages of each single phase input. The invention further includes a means for generating an output signal to interrupt the means for controlling electric current flow when each of the sensed instantaneous voltages are in a specified voltage range.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.